1. Technical Field of the Invention
This invention relates to an image reader and an image reading method.
In particular, this invention is directed to a movable transparent member for improving the focusing of reflection and translucent originals.
2. Description of the Related Art
One type of image reader for reading an original of an image, text, etc., is a flat bed type comprising an original document bed made of a transparent plate of glass, etc., placed on the top of a box-type cabinet. A drive moves a carriage in parallel with the original document bed and the carriage is placed in the cabinet. A light source for reading an opaque (reflection) original, such as paper, and a line sensor comprising an arrangement of a large number of photoelectric conversion elements, are mounted on the carriage. Application light of the light source on the carriage is reflected on the surface of the original on the original document bed and is gathered through a condensing lens on the line sensor.
When an original is read, light is applied from the light source to the original placed on the original document bed and the reflected light from the original is gathered through the condensing lens on the line sensor. While the carriage is moved in a direction perpendicular to the arrangement of the elements of the line sensor and in parallel with the original face, light and shade are detected and are converted into an electric signal. In such an image reader, to read an original in a wide range at high resolution by a small-sized line sensor and miniaturize the whole reader, it is a common practice to reflect light from the original by a number of mirrors placed on a carriage and guide the reflected light into a condensing lens, thereby enlarging an optical path length.
A surface light source is placed above the original document bed and light applied from the surface light source and passed through an original is read by the line sensor on the carriage, whereby a translucent original such as a film can also be read.
However, an image reader for both reflection and translucent originals generally is provided by adding a light source for translucent originals to an image reader for reflection originals, thus, the focus of a condensing lens of a carriage is set so as to match a reflection original. For the reflection original, distance DR, the distance from the surface of an original document bed 3 on the original side thereof to an original 51, is 0.3 mm to 0.4 mm, as shown in FIG. 6. Therefore, the focus of the condensing lens of the carriage is set so as to match a position shifted about 0.3 mm to the original side from the original document bed 3. In this case, the focus depth of the condensing lens is about xc2x10.5 mm, thus focus is obtained if the distance from the surface of the original document bed 3 on the original side thereof is in the range of 0 to 0.8 mm or so.
In contrast, if the original is a translucent original 52 such as a positive film as shown in FIG. 7, the translucent original 52 is stored in a film holder 53 and is further stored in a folder cartridge 54 of an image reader 1, thus distance DP from the surface of the original document bed 3 on the original side thereof to the original 52 becomes about 2.5 mm. Therefore, focus on the translucent original 52 cannot be obtained with the condensing lens focusing on reflection originals; thus, the original is read in a state in which it is slightly out of focus.
However, recently the performance of an image reader has improved to have a resolution of 600 dpi, 1200 dpi, or more; thus the resolution of the image reader is affected by a slight out-of-focus phenomenon.
To remove the out-of-focus condition caused by different original positions, an image reader is used wherein either or both of a photosensor and a condensing lens placed on a carriage and a cabinet are moved in the optical axis direction, whereby the position of the focus of the condensing lens or the optical path length from an original to the condensing lens can be changed seamlessly for always focusing on the original; however, the structure of the image reader is complicated and manufacturing costs are increased.
It is therefore a main object of the invention to provide an image reader and an image reading method capable of focusing a lens on more than one position.
It is another object of the invention to provide an image reader and an image reading method capable of providing a simple structure and reduced manufacturing costs.
According to the invention, an image reader has a transparent member placed so that the transparent member can be inserted into an optical path on which light from an original travels. Thus, the lens focus position when the transparent member exists on the optical path differs from that when the transparent member does not exist on the optical path. Therefore, the lens can be focused on more than one position, so that if the position of an original placed on an original document bed changes, the lens can be focused on the original. Since the transparent member is simply placed in the optical path, the structure is simple and the manufacturing costs can be reduced.
Since the transparent member is placed in the optical path between the original document bed and the lens, the transparent member does not require high accuracy as compared with the case where the transparent member is placed in the optical path between the lens and a photosensor, for example.
Therefore, the transparent member is easily manufactured and the manufacturing costs can be reduced.
Also, the transparent member may be placed in the proximity of the original document bed. Thus, the transparent member can be placed in the portion with a small optical path width in the perpendicular direction to the light travel direction.
Therefore, the transparent member can be miniaturized and the manufacturing costs can be reduced.
Also, the transparent member may be shaped like a plate, so as to have a smooth surface and uniform thickness. Thus, irregular reflection on the surface of the transparent member is prevented and rectilinear propagation of light can be provided, and if the transparent member is inserted into the optical path, necessary resolution can be provided. Since the shape of the transparent member is simple, work is easy and the manufacturing costs can be reduced.
The transparent member may have antireflection means. The antireflection means prevents light passing through the transparent member from being reflected on the surface and the rear face of the transparent member. Therefore, occurrence of a ghost can be prevented. (The ghost means that light from an original is reflected on the surface or the rear face of the transparent member and the original is read as if another image existed at a position different from the position corresponding to the actual image.)
Also, the transparent member is not inserted into the optical path when light reflected on the surface of the original is read. Thus, for example, if the optical path length when light passing through the original is read differs from that when light reflected on the surface of the original is read and the focus position of the condensing lens differs, the lens can be focused on the original if the position of the original placed on the original document bed changes.
According to the image reading method of the invention, to read a translucent original allowing light to pass through, such as a negative film or a positive film, light is applied from above the translucent original, namely, from the side opposite to the photosensor with the original document bed between, and light passing through the translucent original can be read. When a translucent original is read, the transparent member is inserted into the optical path. Therefore, for example, not only light from a reflection original reflected on the surface of the original placed on the original document bed, but also the translucent original placed above the original document bed, can be read. The transparent member is inserted into the optical path, whereby if the position of an original placed on the original document bed changes depending on whether the original is a reflection original or a translucent original, the lens can be focused on the original.